REPRODUCTION 377 



with sperms (male gametes) and the other a female sex organ 

 containing an egg (female gamete). 



In the higher plants, where the sporophyte is the conspicuous 

 generation, the sexual differences in the gametophytes have been 

 thrust farther and farther back into the sporophyte generation. 

 In the more advanced Pteridophytes, the spores which give rise 

 to the male and female gametophytes become differentiated into 

 mega (female) and micro (male) spores. Ultimately the leaves 

 (sporophylls) which bear these spores become differentiated into 

 mega- and micro-sporophylls, and it is in this situation we now 

 find the seed plants with their pistils (megasporophylls) and sta- 

 mens (microsporophylls). In most plants, the stamens and pistils 

 are in the same flower, in which case the flower is said to be per- 

 fect. In other cases, as the pine, corn, oak, cucumber, and wal- 

 nut, the stamens are in flowers on one part of the plant, while 

 the pistils are in other flowers on the same plant. Such plants 

 are said to be monoecious. In still other species (date palm, hop, 

 willow, and poplar), the staminate and pistillate flowers are on 

 different plants, which are then said to be dioecious. From the 

 point of view of sex evolution, these plants are the most advanced, 

 but relatively few plants have reached this degree of differentia- 

 tion. Animals have gone through these same stages of evolution, 

 but have made more progress, with the result that all of the higher 

 animals have lost completely the power of asexual reproduction, 

 and male and female gametes are produced by separate individuals. 

 Perfect forms among the animals are said to be hermaphroditic. 



Origin of Sex.— Why sexual differentiation should have evolved, 

 why it had survival value, is a question that has been given much 

 thought and speculation. Its chief advantage seems to have been 

 that through the union of two germ plasms in sexual reproduction 

 and the resulting combination of characters which results from 

 crossing, there has been the possibility of greater variation, and 

 nature has had more forms from which to select the survivors. 

 There was a greater chance for happy combinations of characters, 

 which would result in forms especially fitted to survive the mani- 

 fold conditions of the world. There has thus been the greatest 

 progress in those places where there was the greatest diversity of 

 mixing germ plasms. Vegetative or asexual reproduction is a 

 quantitative method which may result in the rapid multiplication 

 of a species, but sexual reproduction is a qualitative one which, 



